Multi-stage hydrocatalytic processes for preparing lubricating base oils are known in the art. Examples of such processes are disclosed in British Patent Specification No. 1,546,504, European Patent Specification No. 0,321,298 and U.S. Pat. Nos. 3,494,854 and 3,974,060. From these disclosures it becomes apparent that the first stage of a two stage hydroconversion process is usually aimed at removing nitrogen- and sulphur-containing compounds present in the hydrocarbon oil feed and to hydrogenate the aromatic compounds present in the feed to at least some extent. In the second stage the aromatics content is subsequently further reduced by hydrogenation and/or hydrocracking, whilst hydroisomerization of waxy molecules present in the first stage effluent often takes place as well. The hydrotreatment catalysts used in first and second stage should accordingly be able to adequately serve their respective purposes. From the aforementioned prior art documents it becomes clear that first stage catalysts normally comprise a Group VIII non-noble metal component and a Group VIB metal component on a refractory oxide support. First stage catalysts generally applied, then, include nickel-molybdenum, nickel-tungsten or cobalt-molybdenum on an alumina, silica-alumina or fluorided alumina support.
The patent specifications listed above disclose a variety of suitable second stage catalysts and process conditions to be applied in the second stage, whereby type of catalyst and process conditions are determined by the type of treatment envisaged.
In British Patent Specification No. 1,546,504, for instance, an acidic second stage catalyst is disclosed containing one or more Group VI metal components and one or more non-noble Group VIII metal components, whereby second stage process conditions are relatively severe and include a temperature of between 350 and 390.degree. C. and a pressure of between 50 and 250 kg/cm.sup.2. Operating the second stage under these conditions is likely to cause a substantial degree of aromatics hydrogenation, but also, given the acidic nature of the catalyst employed, a substantial amount of cracking reactions to occur. This inevitably affects the final oil yield due to the formation of a relatively high amount of gaseous components. It would therefore be advantageous if the second stage could be operated at less severe conditions.
U.S. Pat. No. 3,494,854 discloses a second stage hydroisomerization-hydrocracking catalyst comprising a calcium-exchanged, crystalline aluminosilicate (i.e. zeolite) support and a platinum group metal component. Here, the second stage is operated at more severe conditions than the first stage and these second stage operating conditions include temperatures of from about 455.degree. C. to 540.degree. C. and pressures of from about 20 to 140 bar. In the first stage nitrogen level and anyhow sulphur level of the feed are brought down in order not to poison too quickly the second stage catalyst, which normally is not sulphur-resistant. Some hydrocracking may already take place in the first stage, but mostly non-waxy molecules are cracked, since the pour point of the feed does not decrease substantially in the first stage as can be clearly seen from Example 1 of said specification. In the second stage further decrease of the nitrogen level and hydroisomerization and hydrocracking of waxy molecules should take place in order to lower the pour point. However, operating the second stage at such severe conditions will inevitably lead to formation of gaseous components, which goes at the expense of the yield of the final base oil product. Moreover, if too much hydrocracking of waxy molecules occurs, the viscosity index of the final oil will be seriously affected. It would, therefore, be advantageous, if the second stage could be operated at less severe conditions.
In U.S. Pat. No. 3,974,060 a second stage catalyst is disclosed comprising a faujasite support and a noble metal hydrogenation component. The second stage is disclosed to be operated at less severe temperature conditions than the first stage, that is, at a temperature between about 230 and 340.degree. C., and at a pressure of from about 105 to 345 bar in order to limit the amount of cracking that may occur. Conversion of aromatics into polynaphthenics is envisaged to be maximized in the first stage. In the second stage, conversion of polynaphthenics into single ring naphthenes and hydroisomerization of normal paraffins into branched structures are the processes envisaged. Between both stages a gas-liquid separation step may be included to remove any by-product ammonia, hydrogen sulphide and/or light hydrocarbons present in the first stage effluent. A subsequent solvent dewaxing step is considered to be necessary to arrive at a pour point which is appropriate for lubricating base oils.
In European Patent Specification No. 321,298 a hydroisomerization catalyst comprising a noble metal component on a halogenated refractory oxide support is disclosed as the second stage catalyst in a wax isomerization process. Isomerization conditions here include temperatures of from 280 to 400.degree. C. and hydrogen pressures from about 35 to 205 bar. The process disclosed aims at converting slack waxes by isomerizing a substantial portion of the waxy molecules present therein. As the slack waxes by definition have a very high wax content, the viscosity index of the isomerate is very high, usually above 140. After isomerization, the isomerate is fractionated and the lube oil fraction (usually the 330.degree. C.+ fraction and more suitably the 370.degree. C.+ fraction) is subsequently subjected to a dewaxing treatment to attain the required pour point reduction.
Although the processes described above may perform satisfactorily in many respects, it was felt that there is still room for a further improvement, particularly in terms of obtaining lubricating base oils of constant and high quality by means of an efficient and reliable process starting from a distillate feedstock. The present invention provides such a process as can be evidently seen from its advantageous characteristics.
For instance, one advantage of the process according to the present invention is that it yields lubricating base oils of constant and high quality with a high degree of flexibility as to the exact base oil product to be produced. With the present process, namely, it is possible to prepare motor oils, industrial oils and even technical white oils, which base oils predominantly differ from each other in that they have different specifications for contents of aromatics. Another advantage of the present process is that hydrocarbon feedstocks containing relatively high amounts of impurities, such as sulphur- and nitrogen-containing compounds, can be effectively treated and converted into high quality lubricating base oils having excellent VI properties. Yet another advantage is that a very effective use is made of the hydrogen required in the hydrocatalytic conversion stages.